Micropumps can be used in particular for the in situ administration of medicaments, the miniaturization of the pump making it possible to implant them permanently in the body. Using these pumps, small quantities of liquid to be injected can be accurately metered.
Micropumps of this type are in particular described in the article "A piezoelectric micropump based on micromachining of silicon" by H. van Lintel et al. which appeared in Sensors and Actuators, No. 15, 1988, pages 153-157. These micropumps substantially comprise a stack of three wafers, i.e. a wafer of silicon arranged between two wafers of glass.
The wafer of silicon is etched to form a cavity which, together with one of the glass wafers, defines the pumping chamber, at least one inlet valve and at least one outlet valve enabling the pumping chamber to communicate with one inlet channel and one outlet channel respectively. The part of the glass wafer forming a wall of the pumping chamber can be bent by a control element composed, for example, of a piezoelectric disc. This is provided with two electrodes which, when connected to a source of electrical potential, cause the disc to bend and, consequently bend the glass wafer, causing a variation in the volume of the pumping chamber. The flexible wall of the pumping chamber can therefore be displaced between a first position, in which it is relatively far from the opposing wall when the piezoelectric disc is not subjected to any electrical potential, and a second position in which it is closer to the opposite wall when a potential is applied between the electrodes of the piezoelectric disc.
The micropump operates in the following manner. When no electrical potential is applied to the piezoelectric disc, the inlet and outlet valves are in the closed position. When an electrical potential is applied, the pressure inside the pumping chamber increases, causing the outlet valve to open as soon as the pressure in the chamber is greater than the sum of the pressure in the outlet channel and the pressure created by the pre-tension of the valve. The fluid contained in the pumping chamber is then forced towards the outlet channel by the displacement of the flexible wall from the first position towards the second position. During this phase the inlet valve is kept closed by the pressure prevailing in the pumping chamber.
In contrast, the pressure in the pumping chamber falls when the electrical potential is reduced. This closes the outlet valve as soon as the pressure in the pumping chamber is lower than the sum of the pressure in the outlet channel and the pressure created by the pre-tension of the valve, and opens the inlet valve as soon as the sum of the pressure in the pumping chamber and the pressure created by the pre-tension of the valve is less than the pressure in the inlet channel. Fluid is then sucked into the pumping chamber via the inlet channel as a result of the displacement of the flexible wall from the second position towards the first position.
As has already been stated, these micropumps are used in particular for the administration of medicaments. It is therefore important for the output of the micropump to be well determined so that the medication to be injected can be metered in a very precise manner. However, conventional micropumps have certain defects in this respect.
The output of the micropump depends on the variation in the volume of the pumping chamber between the two positions of the flexible wall. This variation in volume depends on various parameters, including the electrical potential applied to the piezoelectric disc and the physical characteristics of the piezoelectric disc (thickness, diameter, dielectric constant) and of the flexible wall (material, thickness). The same electrical potential applied to seemingly identical micropumps could cause differing bending of the pumping chambers of these micropumps which would consequently have different outputs.
The output from one and the same micropump could, moreover, also change in the course of time due to ageing of the materials. Finally, the output of the micropump depends on the pressure in the outlet channel, since the outlet valve only opens when the pressure in the pumping chamber is greater than the sum of the pressure in the outlet channel and the pressure created by the pre-tension of the valve.
In the above mentioned article, H. van Lintel et al. describe a micropump provided with an additional valve which makes it possible to render the output less dependent on the pressure in the outlet channel. However, this micropump does not overcome the other disadvantages mentioned earlier.